1. Field of the Invention
The present invention relates to a hydraulic control apparatus for controlling the speed ratio of a continuously variable transmission for a motor vehicle.
2. Discussion of the Prior Art
A motor vehicle is known, including a continuously variable transmission whose speed ratio is continuously variable, and a clutch device disposed in series with the transmission in a power transmitting line connecting an engine and drive wheels of the vehicle. The clutch device is adapted to disconnect the power transmitting line when a shift lever is operated to a neutral position. In this type of motor vehicle, the continuously variable transmission is controlled such that the speed ratio (speed of the input shaft to the speed of the output shaft) increases toward the maximum value when the amount of the throttle valve opening of the engine decreases. Accordingly, where the transmission is brought to a forward drive position after an inertial or coasting run of the vehicle with the clutch device placed in the neutral position, the engagement of the clutch device is effected when there exists a relatively large difference between the speeds of the input and output shafts of the clutch device. This results in a sudden rise in the engine speed, causing an engaging shock of the clutch device.
To reduce the above-indicated engaging shock of the clutch device upon shifting of the transmission to the drive position after the clutch device is once placed in the neutral position, it is proposed to control the speed ratio of the continuously variable transmission depending upon the vehicle speed during coasting or inertial running of the vehicle with the clutch device in the neutral position, in order to reduce the difference between the speeds of the input and output shafts of the clutch device at the time of shifting of the transmission to the drive position. An example of a hydraulic control apparatus adapted to control the transmission as described above is disclosed in laid-open Publications Nos. 60-231062 and 61-99756 of unexamined Japanese Patent Application.
The above known hydraulic control apparatus for the continuously variable transmission uses a throttle sensor for detecting the throttle valve opening, a vehicle speed sensor for detecting the running speed of the vehicle, a shift position sensor for detecting the currently selected position of the shift lever, a computer for processing the output signals of these sensors, and a solenoid-operated valve device which is controlled by the computer to control the speed ratio of the transmission. Further, the computer also operates to control the transmission during a coasting or inertial running of the vehicle. Accordingly, the hydraulic control apparatus requires the various sensors and a relatively sophisticated electronic control arrangement and tends to be expensive.